Creating a glass like material without it being transparent
You may achieve it using the Backfacing output of a Geometry node as a factor for mixing the Diffuse and Glossy nodes.


Set up the material nodes as pictured below.


Remember to set the Diffuse shader color to pure white and the Glossy shader Roughness value to 0.000. In the .gif above I added a blueish tint to the Glossy shader's color to make the glass plane visible, but if you want to recreate the look of the Glass shader set up the color to pure white.